U.S. Pat. No. 5,505,697 discloses an electrically powered jet injector impacting a plunger driver against a plunger to generate a high initial pressure pulse for piercing the skin of a human. The plunger driver then continues to generate a lower delivery pressure until an injection of for example insulin is completed. A microprocessor within the injector controls the speed and direction of the electric motor which moves the plunger driver. The injector provides a subcutaneous injection especially useful for injecting insulin to treat diabetes.
U.S. Pat. No. 5,505,697 discloses that during the initial pierce phase of the injection, there is a very fast pressure rise to a peak pressure of about 4,000 psi. This peak pressure serves to pierce the skin and generate a short, slightly elongated channel in the sub-cutaneous tissue. At about the time the initial pressure peak is reached, a CPU provides new commands to a power switching circuit board so as to reduce the motor speed in order to deliver a steady state delivery pressure during the delivery phase. The delivery pressure is typically between about ½ and ⅓ of the peak pressure achieved. High pressures are necessary only for a very short duration, to pierce the skin and make a nominal channel into the underlying tissue. Then, the following low, steady delivery pressure only fills the existing channel with fluid, without further tissue disruption. Control of pressure in the delivery phase is provided by closed loop regulation of the motor speed. Specifically, pulse width modulation (PWM) is used, wherein the “on-time” of a fixed frequency pulse generator in the CPU is varied to modulate the average current provided to the motor. The closed loop control allows the device to compensate for variations in battery voltage, friction due to wear or environment temperature changes, or variations in ampoule parameters, especially plunger friction.
There is in U.S. Pat. No. 5,505,697 no mentioning of how to compensate for variations in the battery voltage. Thus, there is in U.S. Pat. No. 5,505,697 no mentioning of how the speed of the motor is kept constant is situations where the battery voltage decreases to levels below the nominal voltage level of the battery. Hence, there is a need for a method for ensuring that the motor speed can be kept constant even in situations wherein the battery voltage falls below the nominal voltage level of the battery.
It is an object of the present invention to provide a method for ensuring constant speed of a battery driven motor in an injection device.
It is a further object of the present invention to provide a method for ensuring constant speed of a battery driven motor in situations where the battery voltage is lower than the nominal voltage of the battery.